This replaces the dynamically allocated two-level array in sys_poll with
a dynamically allocated linked list.  The current implementation causes
at least two alloc/free calls, even if only one or two descriptors are
polled.  This reduces that to one alloc/free, and the .text segment is
around 220 bytes shorter.  The microbenchmark that polls one pipe fd is
around 30% faster.  [1140 cycles instead of 1604 cycles, Celeron mobile
1.13 GHz]